Permanent magnet and method of making the same



G. B. JONAS Sept. 8, 1942.

PERMANENT MAGNET AND METHOD OF MAKlNG THE SAME Filed June 29, 1939 170054 70? 63B Jonas firr0 94 y I00 000 900 800 I00 600 500 400 300 200 I00Patented Sept. 8, 1942 PERMANENT MAGNET AND METHOD OF MAKING THE SAMEElndhoven, Netherlands.

Gottfried Bruno Jonas,

assignments,

assignor, by memo to Hartford National Bank and Trust Company, Hartford,

Conn., as trustee Application June 29, 1939, Serial No. 281,988 InGermany December 6, 1938 21 Claims.

This invention relates to permanent magnets and to a method ofmanufacturing them.

The permanent magnet according to the inventlon exhibits the featurethat it consists 01' a Ni-Al-Fe alloy having a cobalt content of 16 to30%, a nickel content of 12 to 20% and an aluminium content of 6 to 11%and that in one (principal) direction (anisotrope) it has a (BHDm valuewhich is at least 2,000,000, for instance 4,000,000 to 5,000,000, andwhich is at least 50%, for instance 100 to 200%, higher than that of apermanent magnet which is made from the same alloy, but whose magneticproperties are at least substantially equal in all directions(isotrope). Since, as is well known, the size and weight of magnets aredependent upon the (BH)msx value of the magnet steel used, theadvantageous results obtained by the present invention will be readilyappreciated.

The magnets according to the invention preferably have a cobalt contentof about 20% to 25%, a nickel content of about 13% to 17% and analuminum content of about 7% to 10%, and with such compositions it ispossible to obtain (BH)nmx values greater than 3,000,000. I have foundthat such high (BI-I) max values are due to the pronounced arched shapeof the demagnetization curves and, as is well known, the fullness factormma:

B,XHc is decisive for this shape. Magnets according to the inventionhaving a (BH)max value of at least 3,000,000 have a fullness factor ofat least 0.45, generally 0.50 or more.

The magnets according to the invention can be made by subjecting thealloys to the action of a magnetic field. during the cooling requiredfor magnetic hardening.

Figs. 1 and 2 are demagnetization curves of permanent magnets accordingto the invention.

It is already known that the magnetic properties of certainierro-magnetic materials having a high permeability are greatlyinfluenced when, during the thermal treatment, the cooling from hightemperatures is eilected in a magnetic field.

This efiect has also been examined with alloys for permanent magnets(see periodical "Nature" dated July 30, 1938, page 209). This periodicalrefers to an iron alloy for permanent magnets of the Ni-Al type whichconsists of 18% Ni, 10% Al, 12% Co, 6% Cu and 54% Fe and is cooled iroma temperature of 1200 C. in a magnetic field. In this case animprovement in the remanence and in the (3mm value is obtained. Theremanence, for instance, was raised from about 7350 to about 7900Gauss'and the (BH)max from 1,500,000 to 1,800,000. 0

As a possible explanation oi-the influence of the magnetic field it ispointedout that when ferro-magnetic materials are not subjected to amagnetic field during the cooling operation, they.

show the same magnetic properties in all directions. When using amagnetic field during the cooling .operation the material exhibitsanisotropy in such manner that, after subsequent magnetisation in adirection corresponding to the direction of the magnetic field duringcooling, the magnetic properties in this direction are improved whereasthe magnetic properties in other directions are reduced.

It appears from the considerations stated in this respect in theabove-mentioned periodical that the said process of cooling in amagnetic field yields an improvement of about 7% in remanence and of 20%in (BH)max for magnet steel, but that surprising improvements are not tobe expected in this field. This tallies with the prevailing theoreticalconceptions and with the fact that most of the alloys used in practicedo not show at all or only slightly show this effect.

I have found that treatment in a magnetic field yields quite uneimectedresults and more particularly permits one to obtain (B3): values up toabout 5,000,000, when used for the alloys stated in the preamble. It isremarked in this respect that up until now it has been possible only inspecial cases to produce a permanent magnet having a (BH)mu valueexceeding 2,000,000. None oi the commercial magnets have such values,but have values ranging between about 1.8 10 and 1.9x10'.

The progress according to the invention is more surprising, because agroup of Ni-Al-Fe alloys is used which is very uneconomical and henceunusual in practice viz. alloys having a high cobalt content (more than16%), while at the same time I obtain permanent magnets aifordingmaximum economy. The permanent magnets according to the invention mustbe magnetized subsequent to the cooling in the direction oi the lines ofmagnet force used to magnetize the magnet during the cooling, i. e. thelines of force during the subsequent magnetization should substantiallycoincide with the magnetic lines of force used during the cooling. Ifthe magnetic lines of force during the subsequent magnetization wereperpendicular to the direction of the lines of force used during thecooling, the high (BH)max value of the present invention would not beobtained. when I refer to the improvement in (BI-I) value of a permanentmagnet according to the invention, I mean the improvement over apermanent magnet of the same composition, but which is subjected to amagnetization only after the heat treatment. The magnetic fieldpreferably has an intensity of more than 3,000 Gauss.

The cooling operation may be effected in a usual manner which is bestsuited for obtaining favorable magnetic and mechanical properties, andone skilled in the art can select that cooling method which is bestsuited for use with a certain type of alloy when the cooling is effectedwithout the application of a magnetic field. I employ the same coolingmethods when applying the magnetic field during the cooling.

Furthermore it has been found advisable that the action of the magneticfield should take place at least in a temperature-interval which extendsfrom the Curie-temperature to about 150 C. below this temperature andwhich is a part of the temperature-interval traversed during the coolingof the alloy necessary for hardening. I have found, that the time duringwhich the temperature of the alloy remains in this interval between theCurie-point and the temperature 150 C. below this point has an influenceon the magnitude of the effect. From this it follows that the process isparticularly suitable for alloys with which, during the cooling stage ofthe treatment hitherto used, the time during which the temperature ofthe alloy remains within this interval has a certain minimum duration,e. g. 30 seconds, for optimum magnetic properties. I have found that inthese cases the average cooling speed in the usual cooling operation,say between 1,250 C. and 600 C., should be low, i. e. about 10 C. persec. at the utmost, and preferably about 1 C. to C. per see. Inconnection with the internal change of the alloy necessary for theoccurrence of the effect, during the action of the magnetic field, it isadvantageous to use alloys having a high Curie-point, for instancehigher than about 780 C. The above-described alloys containing more than16% Co can satisfy this condition.

As regards the aluminium content it is to be remarked that this shouldbe adapted by a judicious choice to the contents of the other elements,and primarily to the nickel content.

In order to obtain excellent results as regards the value of (13K):! itis advisable that the nickel content should not exceed 17%.

The use of copper, which is desirable in many modern magnet-steelalloys, is not essential and a high content, 1. e. above 7%, is eveninadvisable. However, I have found that in obtaining extremely high (BH)max values it is advisable to admit small quantities of copper, forinstance In the following Table I are given several examples from whichappears that the whole above-mentioned field of alloys can be split upinto sections each of which shows particular magnetic properties.

Table I No. L. Percent negligence 521:; ID-l" ,m-IZ, (I!) 470-765 2.5-4. 2X10 9, 3(I)-ll, 250 510-71) 3-4Xl0' From this table, which isbased on the results obtained from about test-series, it clearly appearsthat the region of alloys designated by a is preferably to be used whenvalue is attached to extremely high values of the remanence. The kind ofalloys designated by b, which contains Ni, Al and Co and in addition Tishould he used if it is attempted to raise the coercive force to thehighest possible extent. It is to be remarked that both in the case aand in the case b the (BI-Um obtained with the largest values of theremanence and of the coercive force respectively has at the same timethe remarkable value of about 3.5)(10 and 4.2)(10 respectively.

The kind of alloy denoted by c, which in addition contains copper withrespect to the lastmentioned alloy, may also be used for this purpose,and the alloy 41 (without titanium) permits more particularly maximumvalues and this jointly with remarkable values both for the remanenceand for the coercive force.

A large number of elements other than those mentioned up till now may beadded in considerable quantities as additional alloy constituents suchthat in each instance the stated improvement of at least 50% in (BH)mnxis obtained. However, a distinction must be made between those metalswhich do not influence the anistropy effect, but do affect the magneticproperties of the alloys and those other metals 1 to 3%. Generally it isadvisable to reduce the nickel content in the case of high coppercontents.

Titanium, which has also proved to be such a valuable element forvarious magnetic steel alloys and was present in the best magneticsteels hitherto known, is not an essential element of my alloy, althougha total percentage of Al and Ti of less than about 12%, the Alpercentage being from 6% to 11%, may be used in my alloy.

However, a high titanium content, for instance more than about 5%, hasproved to be detrimental.

of the periodic system which are usually present in permanent magnetsand which affect both the anistropy effect and the magnetic propertiesof the alloys. As examples of the former metal I might mention, silicon,vanadium, antimony, tin and sulphur, and as examples of the lattermetals I may mention chromium and manganese. More particularly, assumethat a magnet of an Al-Fe-Ni-Co alloy when treated in the usual priorart manner has a (BH)max value of 1,500,000 and when treated in themanner of the present invention has a (BH) max value which, due to theanistropy effect, is double, i. e. 3,000,000. If an element such assilicon, vanadium, antimony, tin and sulphur is added to this alloy andthe alloy is treated in accordance with prior art methods, the (BH)maxvalue will decrease to a value between about 1,000,000 and 1,500,000.However, the anistropy effect is not affected by the addition of theseelements and 7 if the alloy is treated in the manner of the presentinvention, a (1311)., value of about 2,000,000,

a,aas,oss

i. e. approximately double the value obtained when using the prior arttreatment, will be obtained.

If. instead of the above mentioned elements, an element such as chromiumand manganese. is added to the alloy and the alloy is treated in a priorart manner, the (BH)max value will be decreased to about 1,000,000 to1,500,000 but when the alloy is treated by the method of the presentinvention the (B8)... value will be decreased to about 1,400,000, i. e.to a value less than double the value obtained by the prior arttreatment. Additions of other elements such as, for instance, calciumand to a lower degree tungsten may give rise to a slight but clearlyperceptible increase in a characteristic magnetic value, for instance inremanence, but as a rule they tend to influence the other characteristicmagnetic values. It will be understood that neither theanisotropy-eifect nor the magnetic properties are materially influencedby the presence of slight quantitles of other elements, for instance, inthe form of impurities (i. e. in quantities of the order of some tenthsof a per cent).

When making use of the invention one might expect that the highestattainable (BH)max value would occur with alloys whose (311).; value isabout 2,000,000 (which is practically the highest attainable value uptill now) without cooling in a magnetic ileld. To my astonishment I havefound, however, that with these alloys the anisoa high cobalt-contentand which, when heat treated in a manner suitable for obtaining anoptimum (BH)max value, have a remanence of more than 10,000 Gauss, theimprovement in the (8mm value due to the remarkably strong py-eiiect ofthe steel is considerable and that in general the remanence has aconsiderable influence in obtaining high (88)., values. In this manner Ihave found it possible to obtain the unprecedented (BID-m value of morethan 25x10 and even more than x10, while at the same time securing avery high remanence. for instance, more than 12,000 Gauss. Thus, thepresent invention makes it possible to comply in a practical manner withthe frequent desire to have a high (BI-Um together with a highremanence.

In contradistinction to what has been said in the above article in"Nature'. with respect to the change in coercive force I have found thatmore particularly with alloys according to the invention showing thehigher (BIDm values, not only the remanence but also the coercive force,measured in a preferred direction is increased. The enormous. increasein the (3mm value is partly to be ascribed thereto.

In the following Table II are stated, by way of example, various alloys,which have been cooled according to a usual heat treatment (thirdcolumn) and in a magnetic field (fourth column).

Table II Results obtainedthb o timum (an btained by mak in g mal m g g iAvmgo Fuum Composition in use of a usual therg Improve- No. the pgoolnmn, speed factor per cent mal treatment with but in mm the mun mentin out magnetic field, h them 0 In m (13H)... (RH) mean! xmsgneticzl l ds its: between ann g about 1,310 mutant to G10 0 N1 A1 00 Cu Ti (BID-uHo, Ba (BID-u H 3.-

C.p./sec.

1 10.0 8.5 23 1,211,010 9,050 3,450, 402 12,550 1 183 0.55 2 13.5 8 241.5 1,320,000 370 0,450 3,770,)0 505 13,100 2.0 185 0.57 3 13.5 8 84 3LmiXlO 535 8,300 4,790,000 015 12,700 1.8 185 0.00 4 10 7.8 35 2.81,600,000 004 7,000 3,057,000 040 10,000 4 91 0.48 5 14 7.1 24 3 2.41,721000 505 7,900 57%,000 050 11, 4 12) 0.52 0 14 7.5 2) 0.5 1.81,050,000 020 7,350 3,250,000 570 0,825 4. 97 0.49 7. 15.5 8.1 so 1 12.3 1,824,000 640 8,150 3,117,000 585 10,210 4 71 0.45

(BK) max far exceeding the values hitherto attainable by 'means of thebest kinds of steel.

Without cooling in a magnetic field higher values 5f (BH)max generallyinvolved hitherto a lower remanence; for instance, (BI-I values of about2,000,000 could only be attained at comparatively low values of theremanence, for instance not more than 9,000 Gauss. This is due interalia to the fact that the improvement in (EH) max could only be obtainedby raising the coercive force, which increase was obtained to a greateror less extent at the cost of the remanenee.

I have found that with steel alloys which have 7 The average coolingspeed stated in the fifth column was used in both of the said cases andis so chosen that in cooling without magnetic field about the highestattainable product of B and H is attained.

Column 6 gives the percentage improvement of the (BH)max value accordingto column 4 over the (310m value according to column 3. It appearstherefrom that, for instance, a composition according to the examplesnumbered 1 to 3 permits a magnet steel to be obtained which is aboutthree times better than the same steel which has not been cooled in amagnetic field. From the other examples it appears that this compositionyields also an improvement of the (BH)max value of at least 50%. Fromexample 2 according to the invention it appears that incontradistinction to the prior art a very high (BH)m-x may quite well beassociated with a remanence already high in itself.

The accompanying Figures 1 and 2 show the de-magnetisation curves of theexamples numbered 1 and 3 in the table. The curves 1 and 2 correspond toa magnet steel according to the invention, whereas curves 3 and 4correspond to the same steel treated according to the third column ofthe table. For comparison both 11gures represent in dotted lines thede-magnetisation curve of a known modern magnet steel having a very high(BI-1m. It appears from the figures that the high (BI'Dmax of themagnets according to the invention primarily results from a highlyraised remanence and a more arched shape of the de-magnetisation curve,although the improvement of the coercive force with respect to the samesteel cooled without magnetic field (compare curves 1 with 3 and 2 with4) is also material.

For a proper understanding of the commercial importance of the presentinvention, it should be noted that the results given in the above tablesare not limited to magnets of small dimensions or to testpieces.

I claim:

1. A permanent magnet consisting of a ferrous alloy containing about 16%to 30% cobalt, about 12% to 20% nickel, about 6% to 11% aluminum and theremainder principally iron, said magnet being magnetically-anisotropicand having a (BH)max value in the principal direction at least about 50%higher than the (31 1):; value of a substantially magnetically-isotropicpermanent magnet of the same alloy.

2. A permanent magnet consisting of a ferrous alloy containing about 20%to 25% cobalt, about 13% to 17% nickel, about 7% to aluminum and theremainder principally iron, said magnet being magnetically-anisotropicand having 2. (BH) max value in the principal direction which is atleast 2,500,000 and is at least about 50% higher than the (BIDmax valueof a substantially magnetically-isotropic permanent magnet of the samealloy.

3. A permanent magnet consisting of a ferrous alloy containing about 20to cobalt, about 13. to 16.5% nickel, about 7.1% to 8.5% aluminum andthe remainder principally iron, said magnet beingmagnetically-anisotropic and having a (BH)msx value in the principaldirection which is equal to at least about 3,000,000 and which is atleast 50% greater than that of a substantially magnetically-isotropicpermanent magnet of the same composition.

4. A permanent magnet consisting of a ferrous alloy containing about 21%to 25% cobalt, about 14% to 20% nickel, about 8% to 10% aluminum and theremainder principally iron, said magnet being magnetically-anisotropicand having a (BI'Dmax value in the principal direction which is at least2,000,000 and which is at least 50% greater than that of amagnetically-isotropic permanent magnet of the same alloy, and aremanence of at least 8,000 Gausses.

5. A permanent magnet consisting of a ferrous alloy containing about 23%to 24% cobalt, about 13.5% to 16% nickel, about 7% to 8.5% aluminum andthe remainder substantially iron, said magnet beingmagnetically-anisotropic and having a (BHMMX value in the principaldirection of at least 3,000,000.

6. A permanent magnet consisting of a ferrous alloy containing about 16%to cobalt, about 12% to 20% nickel, about 6% to 11% aluminum, theremainder being iron and at least one of the elements titanium in anamount less than about 5% and copper in an amount less than about 7%,said magnet being magnetically anisotropic and having a (BH) max valuein the principal direction of at least about 50% higher than the (BH)mxvalue of a substantially magneticallyisotropic permanent magnet of thesame alloy.

7. A permanent magnet consisting of a ferrous alloy containing about 20%to 25% cobalt, about 13.5% to 16.5% nickel, about 7.1% to 8.1% aluminum,the optional inclusion of one or both of the elements copper in anamount less than about 6.5% and titanium in an amount less than about2.8% and the remainder iron, said magnet being magnetically-anisotropicand having a (BH)mu value in the principal direction which is at least3,000,000 and which is at least 50% greater than that of amagnetically-isotropic permanent magnet of the same alloy, and aremanence of at least 8,000 Gausses.

8. A permanent magnet consisting of a ferrous alloy containing about 23%to 24% cobalt, about 13.5% to 16% nickel, about 7% to 8.5% aluminum, theremainder being iron and small quantities of at least one of theelements copper and titanium, said magnet being magneticallyanisotropicand having a (BH)mn value in the principal direction of at least3,000,000.

9. A permanent magnet consisting of a ferrous alloy containing about 16%to 30% cobalt, about 12% to 20% nickel, about 6% to 11% aluminum, notmore than 5% titanium and the remainder principally iron, the totalaluminum-titanium content being not more than about 12%, said magnetbeing magnetically-anisotropic and having a (BH)max value in theprincipal direction at least about 50% higher than the (BHhnu value of asubstantially magnetically-isotropic permanent magnet of the same alloy.

10. A magnetically-anisotropic permanent magnet consisting of a ferrousalloy containing about 14% to 18% nickel, about 22% to 25% cobalt, about6% to 8% aluminum, about 0.4% to 4% titanium and the remainderprincipally iron, said magnet having a coercive force of more than 450Oersteds and a (BH)max value which is greater than 2,500,000 and whichis at least 50% greater than that of a substantiallymagneticallyisotropic permanent magnet of the same composition.

11. A magnetically-anisotropic permanent magnet consisting of a ferrousalloy containing about 14% to 18% nickel, about 22% to 25% cobalt, about6% to 8% aluminum, about 0.4% to 4% titanium and the remainderprincipally iron, said magnet having a coercive force of more than 450Oersteds and a (BH)max value which is greater than 3,000,000 and whichis at least 50% greater than that of a substantiallymagneticallyisotropic permanent magnet of the same composition.

12. A permanent magnet consisting of a ferrous alloy containing about 20to 27.5% cobalt, about 12% to 15% nickel, about 8% to 8.5% aluminum,about 1.5% to 6.5% copper and the remainder principally iron, saidmagnet being magnetically-anisotropic and having a (BI-Um value in theprincipal direction which is equal to at least about 3,500,000 and whichis atleast 50% greater than that of a substantiallymagnetically-isotropic permanent magnet of the same composition.

13. A magneticallyanisotropic permanent magnet consisting of a ferrousalloy containing about 13% to 16.5% nickel, about 18.5% to 25% cobalt,about 6.5% to 8.5% aluminum, about 1% to 3% titanium, about 1% to 7%copper and the remainder principally iron, said magnet having a (BH)maxvalue which is greater than 3,000,000 and which is at least 50% greaterthan that of a substantially magnetically-isotropic permanent magnet ofthe same composition.

14. In the manufacture of a permanent magnet the steps of forming a bodyof a ferrous alloy containing about 16% to 30% cobalt, about 12% to 20%nickel, about 6% to 11% aluminum and the remainder principally iron, andsubjecting the body to a magnetic field during the cooling operationrequired for magnetic hardening.

15. In the manufacture of a permanent magnet, the steps of forming abody of a ferrous alloy containing about 16% to 30% cobalt, about 12% to20% nickel, about 6% to 11% aluminum, and the remainder principallyiron, subjecting the body to a magnetic field during the coolingoperation required for magnetic hardening to thereby make the alloymagnetically-anisotropic, and subsequently magnetizing the body in adirection corresponding to the direction of magnetization during coolingto thereby obtain a (BH)max value at least 50% higher than that obtainedwith the same alloy without magnetization during the cooling.

16. In the manufacture of a permanent magnet, the steps of forming abody of a ferrous alloy having a Curie-temperature above about 780 C.and containing about 16% to 30% cobalt, about 12% to 20% nickel, about6% to 11% aluminum and the remainder principally iron, subjecting thebody to a magnetic field during the cooling operation required formagnetic hardening and while the alloy is at a temperature between itsCurie-temperature and a temperature lying about 150 below theCurie-temperature to thereby make the alloy magnetically-anisotropic,and subsequently magnetizing the body in a direction corresponding tothe direction of magnetization during cooling to thereby obtain a (BB)max value at least 50% higher than that obtained with the same alloywithout magnetization during the cooling.

17. In the manufacture of a permanent magnet, the steps of forming abody of a ferrous alloy containing about 20% to 25% cobalt, about 13% to17% nickel, about 7% to aluminum and the remainder principally iron,subjecting net, the steps of forming a body of a ferrous alloycontaining about 20% to 25% cobalt, 13.5% to 16.5% nickel, 7.1% to 8.5%aluminum and the remainder principally iron, subjecting the body tomagnetization during the cooling operation required for magnetichardening, and subsequently magnetizing the body in a directioncorresponding to the direction of magnetization during the cooling tothereby obtain a (BH) max value which is greater than 3,000,000 andwhich is at least 50% greater than that of a substantiallyisotropicpermanent magnet of the same composition.

19. In the manufacture of a permanent magnet, the steps of forming abody of a ferrous alloy containing about 23% to 24% cobalt, about 13.5%to 16% nickel, about 7% to 8.5% aluminum and the remainder principallyiron, subjecting the body to magnetization during the cooling operationrequired for magnetic hardening and subsequently magnetizing the body ina direction corresponding to the direction of magnetization during thecooling to thereby obtain a (BH) max value greater than 3,000,000.

20. In the manufacture of a permanent magnet, the steps of forming abody of a ferrous alloy containing about 20% to 25% cobalt, about 13.5%to 16.5% nickel, about 7.1% to 8.5% aluminum, the optional inclusion ofone or both of the elements copper in an amount less than about 6.5% andtitanium in an amount less than about 2.8% and the remainder iron,cooling the body in a magnetic field from a temperature of about 1200 C.to a temperature of about 600 C. at a rate of between about 1 and 4.3degrees centigrade per second to thereby make the alloymagneticallyanisotropic, and subsequently magnetizing the body in adirection corresponding to the direction of magnetization during coolingto thereby obtain a (BI-Drum: value at least 50% higher than thatobtained with the same alloy without magnetization during the cooling.

21. In the manufacture of a permanent magnet, the steps of forming abody of a ferrous alloy containing about 23% to 24% cobalt, about 13.5%to 16% nickel, about 7% to 8.5% aluminum, the remainder being iron andsmall quantitles of at least one of the elements copper and titanium,subjecting the body to magnetization during the cooling operationrequired for magnetic hardening and subsequently magnetizing the body ina direction corresponding to the direction of magnetization during thecooling to thereby obtain a (BH)max value greater than 3,000,000.

' GO'I'IFRIED BRUNO JONAS.

